Particles containing an alkali agent such as detergent particles tend to suffer from caking, i.e., such a phenomenon that the particles are bonded together into a solidified state, during storage. In particular, the particles containing an alkali metal carbonate tend to react with carbon dioxide and water in air during storage to form a sesqui-carbonate on a surface thereof. In accordance with researches made by present inventors, it has been found that the thus formed sesqui-carbonate causes aggregation between the adjacent particles which results in occurrence of caking of the particles. Once the particles suffer from caking, there tends to occur such a problem that the resulting detergent not only exhibits a poor appearance but also is incapable of being accurately weighed and, therefore, considerably deteriorated in handing property upon use.
To avoid occurrence of such a caking phenomenon, it is generally known that the surface of the respective detergent particles is suitably coated to prevent the particles from contacting with outside air. However, since the particles such as a powdery detergent are also required to exhibit a promoted dissolution in water, it is very difficult for these particles to satisfy both a good stability and a good dissolvability at the same time.
Patent Document 1 discloses a detergent composition containing a smectite-type clay softening agent. Patent Document 2 discloses a process for producing a granular detergent composition by adding a swelling clay such as sodium montmorillonite to a granular substance containing a detergent active substance and a water-soluble crystalline inorganic salt and then treating the resulting mixture using a high-speed mixer/granulator. The swelling clays described in Patent Documents 1 and 2 are not coated on the surface of the respective detergent composition particles, but incorporated within the particles.
On the other hand, Patent Document 3 discloses a granular chemical agent for detergent compositions which is provided on a surface thereof with a coating containing a lipophilic smectite-type clay. Patent Documents 4 and 5 disclose detergent particles obtained by coating the surface of the respective base detergent particles with a primer agent layer and a surface-coating agent layer.
In the techniques described in Patent Documents 3 to 6 in which the clay mineral is used as a surface-modifying agent for detergent particles, occurrence of caking of the detergent particles can be inhibited only to some extent but not to a fully satisfactory extent. Therefore, there is an increasing demand for development of techniques for improving an anti-caking property of the detergent particles without deterioration of dissolvability in water.    Patent Document 1: JP 49-85102A    Patent Document 2: JP 3-210398A    Patent Document 3: JP 2-286800A    Patent Document 4: JP 2004-143394A    Patent Document 5: JP 2005-171149A    Patent Document 6: JP 2008-189726A